225 research outputs found
Ion-atom hybrid systems
The study of interactions between simultaneously trapped cold ions and atoms
has emerged as a new research direction in recent years. The development of
ion-atom hybrid experiments has paved the way for investigating elastic,
inelastic and reactive collisions between these species at very low
temperatures, for exploring new cooling mechanisms of ions by atoms and for
implementing new hybrid quantum systems. The present lecture reviews
experimental methods, recent results and upcoming developments in this emerging
field.Comment: To appear in the Proceedings of the International School of Physics
Enrico Fermi, Course 189: Ion Traps for Tomorrows Application
Fine- and hyperfine-structure effects in molecular photoionization: II. Resonance-enhanced multiphoton ionization and hyperfine-selective generation of molecular cations
Resonance-enhanced multiphoton ionization (REMPI) is a widely used technique
for studying molecular photoionization and producing molecular cations for
spectroscopy and dynamics studies. Here, we present a model for describing
hyperfine-structure effects in the REMPI process and for predicting hyperfine
populations in molecular ions produced by this method. This model is a
generalization of our model for fine- and hyperfine- structure effects in
one-photon ionization of molecules presented in the preceding companion
article. This generalization is achieved by covering two main aspects: (1)
treatment of the neutral bound-bound transition including hyperfine structure
that makes up the first step of the REMPI process and (2) modification of our
ionization model to account for anisotropic populations resulting from this
first excitation step. Our findings may be used for analyzing results from
experiments with molecular ions produced by REMPI and may serve as a
theoretical background for hyperfine-selective ionization experiments
Cold molecular ions on a chip
We report the sympathetic cooling and Coulomb crystallization of molecular
ions above the surface of an ion-trap chip. N and CaH ions were
confined in a surface-electrode radiofrequency ion trap and cooled by the
interaction with laser-cooled Ca ions to secular translational
temperatures in the millikelvin range. The configuration of trapping potentials
generated by the surface electrodes enabled the formation of planar bicomponent
Coulomb crystals and the spatial separation of the molecular from the atomic
ions on the chip. The structural and thermal properties of the Coulomb crystals
were characterized using molecular dynamics simulations. The present study
extends chip-based trapping techniques to Coulomb-crystallized molecular ions
with potential applications in mass spectrometry, cold chemistry, quantum
information science and spectroscopy.Comment: 5 pages, 4 figure
Superstatistical energy distributions of an ion in an ultracold buffer gas
An ion in a radiofrequency ion trap interacting with a buffer gas of
ultracold neutral atoms is a driven dynamical system which has been found to
develop a non-thermal energy distribution with a power law tail. The exact
analytical form of this distribution is unknown, but has often been represented
empirically by q-exponential (Tsallis) functions. Based on the concepts of
superstatistics, we introduce a framework for the statistical mechanics of an
ion trapped in an RF field subject to collisions with a buffer gas. We derive
analytic ion secular energy distributions from first principles both neglecting
and including the effects of the thermal energy of the buffer gas. For a buffer
gas with a finite temperature, we prove that Tsallis statistics emerges from
the combination of a constant heating term and multiplicative energy
fluctuations. We show that the resulting distributions essentially depend on
experimentally controllable parameters paving the way for an accurate control
of the statistical properties of ion-atom hybrid systems.Comment: Main text 6 pages, 4 figures. Total length including supplemental
material 11 pages, 6 figure
The energy distribution of an ion in a radiofrequency trap interacting with a nonuniform neutral buffer gas
An ion in a radiofrequency (rf) trap sympathetically cooled by a
simultaneously trapped neutral buffer gas exhibits deviations from thermal
statistics caused by collision-induced coupling of the rf field to the ion
motion. For a uniform density distribution of the buffer gas, the energy
distribution of the ion can be described by Tsallis statistics. Moreover,
runaway heating of the ion occurs if the buffer gas particles are sufficiently
heavy relative to the ion. In typical experiments, however, ultracold buffer
gases are confined in traps resulting in localised, non-uniform density
distributions. Using a superstatistical approach, we develop an analytical
model for an ion interacting with a localised buffer gas. We demonstrate
theoretically that limiting collisions to the centre of the ion trap enables
cooling at far greater mass ratios than achievable using a uniform buffer gas,
but that an upper limit to the usable mass ratio exists even in this case.
Furthermore, we analytically derive the energy distribution for an ion
interacting with a buffer gas held in a harmonic potential. The analytical
distribution obtained is found to be in excellent agreement with the results of
numerical simulations.Comment: 27 pages, 6 figure
Superstatistical velocity distributions of cold trapped ions in molecular dynamics simulations
We present a realistic molecular-dynamics treatment of laser-cooled ions in
radiofrequency ion traps which avoids previously made simplifications such as
modeling laser cooling as a friction force and combining individual heating
mechanisms into a single effective heating force. Based on this implementation,
we show that infrequent energetic collisions of single ions with background gas
molecules lead to pronounced heating of the entire ion ensemble and a
time-varying secular ensemble temperature which manifests itself in a
superstatistical time-averaged velocity distribution of the ions. The effect of
this finding on the experimental determination of ion temperatures and rate
constants for cold chemical reactions is discussed.Comment: 13 pages, 12 figures, accepted for publication in Phys. Rev.
Optimised surface-electrode ion-trap junctions for experiments with cold molecular ions
We discuss the design and optimisation of two types of junctions between
surface-electrode radiofrequency ion-trap arrays that enable the integration of
experiments with sympathetically cooled molecular ions on a monolithic chip
device. A detailed description of a multi-objective optimisation procedure
applicable to an arbitrary planar junction is presented, and the results for a
cross junction between four quadrupoles as well as a quadrupole-to-octupole
junction are discussed. Based on these optimised functional elements, we
propose a multi-functional ion-trap chip for experiments with translationally
cold molecular ions at temperatures in the millikelvin range. This study opens
the door to extending complex chip-based trapping techniques to
Coulomb-crystallised molecular ions with potential applications in mass
spectrometry, spectroscopy, controlled chemistry and quantum technology.Comment: 19 pages, 10 figure
Molecules and Ions at Very Low Temperatures
The generation and study of 'cold' gas-phase molecules characterised by very low translational temperatures Ttrans ? 1 K is an upcoming field of research in physical chemistry which has received considerable attention over the past years. A particular interesting
form of cold molecules are ensembles of cold localised cations in ion traps which form ordered structures known as 'Coulomb crystals'. The present article reviews the experimental methods used for the generation of atomic and molecular Coulomb crystals and highlights recent experiments which
take advantage of their intriguing properties in order to study chemical reactions at very low temperatures with single-particle sensitivity
Molecular-ion quantum technologies
Quantum-logic techniques for state preparation, manipulation, and non-destructive interrogation are increasingly being adopted for experiments on single molecular ions confined in traps. The ability to control molecular ions on the quantum level via a co-trapped atomic ion offers intriguing possibilities for new experiments in the realms of precision spectroscopy, quantum information processing, cold chemistry, and quantum technologies with molecules. The present article gives an overview of the basic experimental methods, recent developments and prospects in this field
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